PETROLOGICAL ABSTRACTS AND REVIEWS 633 



associated with pyroxene or hj^Dersthene. This rock we shall distinguish by 

 the name of anorthosite In some cases the .... dark mineral is en- 

 tirely wanting." (Geol. Canada, 1863,22.) According to modern petrographic 

 usage the term anorthosite has come to mean a basic-feldspar rock, which is 

 practically free from dark minerals, say with less than 5 per cent, yet the 

 Canadian anorthosites are spoken of in the sense of Hunt. Bowen, therefore, 

 if he speaks of the whole formation, is incorrect when he says that "Anortho- 

 sites are made up almost exclusively of the single mineral plagioclase," for as 

 MiUer shows, the dark mineral averages 10 per cent. He says, "The main 

 bulk .... contains 5 to 10 per cent of minerals other than plagioclase .... 

 in many places there are 10 to 20 per cent, or even more of dark mineral. It is 

 also true that some portions of the mass contain less than 5 per cent of dark 



minerals Conservatively estimated, I believe the average .... 



anorthosite carries fully 10 per cent of minerals other than plagioclase." 

 Attacking Bowen 's theory that the anorthosite may not have been at one time 

 in a molten condition. Miller says: "The Adirondack anorthosite would have 

 formed a melt of notably more complicated composition than the artificial melt 

 with 10 per cent diopside and [it would have been formed] under deep-seated 

 geologic conditions. Is it safe to say, therefore, that such a melt may not have 

 been a true magma with a high percentage of hquid ? . . . . Another impor- 

 tant consideration is the almost certain presence of very appreciative amounts 



of dissolved vapors, particularly water vapor, in the magma Also the 



presence of about 2 per cent iron oxide in the typical anorthosite should not be 



overlooked All things considered, therefore, I not only think it very 



reasonable to apply the mutual solution theory to the anorthosite, but also to 

 regard the anorthosite to have existed in magmatic condition at a moderate 

 temperature." He says further: "I consider the main steps in the develop- 

 ment of the anorthosite to have been as follows: First, intrusion of a lacco- 

 lithic body of gabbroid magma .... second, relatively rapid cooling of the 

 marginal portion to give rise to the chilled gabbroid border phase; and, third, 

 settling of many of the slowly crystallizing femic minerals in the still molten 

 interior portion of the laccolith, leaving a great body of magma to gradually 

 crystallize into anorthosite. Thus, at the bottom, and probably nowhere 

 visible in the field, lies a mass of pyroxenite or peridotite . ..." (p. 457). 



Miller says in reference to a syenite (p. 438), "Labradorite and andesine are 

 always present and oligoclase usually." The reviewer must again make the 

 statement which he has made a number of times before, that he doubts whether 

 in rocks which are composed of crystals of a single generation, two different 

 plagioclases can occur together except as zonal growth. 



It is impossible in this abstract to give all of Miller's conclusions. Briefly 

 they are: 



The Adirondack anorthosite is a great laccolithic intrusive body — older 

 than the accompanying granite-syenite series. The average rock contains 

 fully 10 per cent of dark minerals, and is differentiated practically in situ from 



